1. Field of the Invention
This invention relates to a method as well as to a device for producing coal dust in a coal grinding and drying system, particularly for firing of a combustion system usable in the manufacture of cement clinker.
2. Prior Art
In conjunction with this invention, the term "calcining" is meant to signify the thermal expulsion of CO.sub.2 from a mineral.
Experience has taught that the following criteria, among others, should be taken into consideration in selecting a location for a coal pulverizing system to supply coal dust to a calcining installation.
1. The transport distances for the raw material or, respectively, finished products,
2. The connection of the coal grinding system to the gas circulation system of the consumer system, particularly for the removal of hot gas with a low as possible content of oxygen for the grinding/drying operation, and
3. The elimination of any coal mill exhaust gas containing combustible materials or gases as well as odoriphores damaging to the environment.
In a combustion system for the manufacture of cement, comprising a pre-heating stage, calcining stage and kiln for clinker burning which is fired with coal dust, the coal grinding system should be erected, if possible, where hot gases with the lowest possible content of oxygen are available. This is because, after the grinding/drying operation in the grinding system, the exhaust gases including water vapor and secondary air added by the grinding system should have an oxygen content of less than 10%.
The exhaust gases withdrawn from suspension type heat exchanger systems with an operating temperature range of 350.degree. C. to a maximum 500.degree. C. exhibit an oxygen content of 2% to a maximum of 5%. Using these gases, as an input to the coal pulverizing system, the exhaust gases produced from the grinding/drying system exhibit usually no more than 10% oxygen.
The introduction of the mill exhaust gases with a temperature range of 80 through 100.degree. C. into the associated combustion system is difficult. This is because the system supplied with the coal dust exhibits process sequences with endothermic reaction enthalpies such as, for example, sintering zones given minerals and ores, melting zones given ores, deacidification, calcining and mineral conversion areas given minerals, cement, limestone, dolomite, magnesite, earths, ores, etc.
As a result, as previously disclosed in the patent application No. P 27 45 424.6, corresponding to U.S. Ser. No. 949,953, now U.S. Pat. No. 4,193,554, it is desirable to integrate a method and a device for the drying and fine-grinding of coal for the powdered coal firings of a combustion system with this combustion system. In this way, operating conditions which are optimum for the integrated system may be determined based on economic feasibility, reliability and operational safety. In particular, any deterioration of the calcining process must be avoided.